This invention relates to cranes and in particular to cranes of the strut type having a jib which is normally of lattice or lightweight construction, which is pivoted to a platform or the like (which may form part of a mobile crane vehicle or which may be mounted in a fixed location), the head of the jib being supported by tie "ropes" to enable the jib to carry the required load and to be "derricked" up and down.
If, which is particularly desirable, when the crane is a mobile crane, the jib of a crane of this type comprises a number of telescopic sections, then when the telescopic sections are moved outwardly to extend the jib, the tie "ropes" must also be extended so as to maintain the jib angle. Otherwise the jib is pulled initially to a more vertical position and, subsequently, further extension of the jib is prevented by the tie "ropes".
Compensation for jib tie "ropes" during telescopic extension of the jib to prevent jib elevation is provided for in our U.K. Pat. No. 1,262,553 (Inventor: Valentine W. Burney) which will be referred to hereinafter as "our former British patent".
In the jib arrangement of our former British patent, the tie rope or derrick rope as it is called in that patent extends from a point adjacent the top or head of the outer section of the jib, around the head and then middle of the derrick mast to the head of the inner section of the jib to pulleys at the foot of the intermediate jib section and thence back to the head of the inner section jib. Thus the intermediate section is partly supported by the jib tie rope when the jib is in its extended position, but only the intermediate section is so supported, and support of the outer section of the jib relies entirely on the telescoping means. As telescoping cranes increase in length so the problems due to weight and stress increase and the problem is now to provide better support for the outer section of the jib when this is extended.